The present invention relates to a hydraulic transmission for varying the speed of an output shaft in comparison to an input shaft and, in particular, to a hydraulic transmission of the type utilized to independently control the speed and rotational direction of multiple wheels on a vehicle.
Many hydraulic transmissions have been previously designed which allow an operator to infinitely vary the output of the transmission or even to reverse the output of the transmission as compared to the input. Normally, this has been accomplished by use of a swash plate which is either manually operated by the operator or hydraulically motivated to vary flow of hydraulic fluid through a rotating pump head having axially moving pistons. The flow of hydraulic fluid from the pump head pistons, in turn, rotates a motor head by motivating a corresponding set of pistons therein acting against a fixed cam and, consequently, rotates an output axle attached to the motor head.
Typically, the prior art transmissions have been provided with various features such as gear reduction, brake setting equipment and the like. Unfortunately, these features are usually provided external of the transmission and significantly add to the volume and mass of the overall apparatus. Applicants determined that it was quite desirable to have an integrated transmission wherein substantially all of these necessary or desirable functions could be provided internally in the transmission, while also producing a highly effective and quite efficient transmission.
In particular, transmissions of this type are frequently used on equipment such as "zero turning radius" lawnmowers and the like wherein a potentially dangerous situation confronts the operator, bystander and the equipment itself if the device us allowed to continue to be propelled should the operator release the controls, as when the operator is inadvertently thrown from the device or becomes injured. Therefore, a "deadman" mechanism is often provided for automatically returning the transmission to a neutral configuration in such a situation such that the device does not continue to be powered, if the controls are released.
Previous transmissions of this type have generally relied on some kind of external device such as counter-operating biasing springs which are designed to return the operator control shaft to a neutral position should the operator release the shaft. External devices of this type can be easily tampered with by a user or damaged. Integration of such a return to neutral function into the transmission itself allows for a reduction in external parts which can be damaged or inappropriately removed and substantially reduces the external structure required to support various features of the transmission.
Another problem frequently encountered in transmissions of this type for use in lawnmowers and the like is that operation tends to be somewhat jerky or bouncy, as the operator is usually unable to smoothly control the transition from one speed to another and often attempts to make abrupt changes. Vibration from such jerky operation has a tendency to wear more heavily on the machine and the operator as well. Consequently, it is desirable to dampen the output of such a transmission to prevent such jerky motion.
Not only is it desirable to be able to have a return to neutral function, as described above, but it is also desirable for the operator to have a positive feel for the neutral position, whether the operator is moving from neutral to forward or from neutral to reverse. This feature is referred to herein as a positive neutral function and, in general, the feature requires that the operator extend more energy or movement in converting from the neutral orientation of the transmission to either a forward or reverse orientation, as compared to the amount of energy expended or movement required to transfer from one speed to another in a particular direction. As with the other features noted above, it is desirable that the structure required to provide this feature be incorporated within the transmission itself.
For reasons of convenience and simplicity, it is preferable that the transmission receive power directly from the motor drive train without passing through gear reduction apparatus. Unfortunately, most output requirements are such that the output rotational speed must be substantially reduced as compared to the input rotational speed taken directly from the motor. Consequently, it is further desirable that the transmission incorporate an internal gear reduction system allowing for a substantial reduction in output speed as compared to input speed, even when the hydrostatic transmission of the type having axially moving pistons driving the output motor is oriented to provide the maximum output speed. (It is noted that at some lower than maximum output speeds, virtually all variable output transmissions inherently provide some gear reduction because of power bypass of one type or another, but none of the prior art transmissions known to applicants provide a substantial internal gear reduction at full output).
Certain of the vehicles utilizing transmissions of this type include brakes which effectively prevent the vehicle from moving when the motor driving the transmission is stopped. These brakes are usually manually applied and require a substantial amount of structure mounted on the vehicle. In particular, parking brakes of this type have usually included an operator lever arm with an associated linkage connecting the lever arm to a brake mechanism which, in turn, locks the drive wheels, drive shaft or the like. Again, to simplify this structure and to integrate the braking function into the transmission, it was deemed desirable by applicants to construct a transmission which would have internal structure to automatically engage a parking brake when the motor driving the transmission was stopped.
While a parking brake of this type is highly advantageous, it is also necessary to provide such a brake with an override structure which will allow the operator to selectively override the brake; for example, where the operator wishes to push the vehicle to a different position when the motor will not start. Further, it is advantageous to have a braking system of this type which allows the operator to selectively engage the brake when the motor is operating. This allows the operator to dismount from the vehicle while the motor is running without fear of the vehicle rolling downhill or the like.
Applicants also deemed it desirable to provide a porting block mountable between the pump head and motor head of the transmission which would incorporate necessary flow channels, check valves and the like to very effectively and simply provide for flow of fluid about the transmission and to eliminate the need for hoses connecting the pump head and motor head. The porting block is also provided with a fluid flow bypass to allow an operator to push an associated vehicle without the hydraulic action of the fluid locking the wheels of the vehicle.
Further desirable features of a unified transmission of the type described herein include that, for certain embodiments the vehicle wheels may be mounted directly on output shafts of the transmission, while transmission driving wheels on opposite sides of the vehicle are driven in common by a single input shaft, also while providing for a gear reduction, as described above, internal of the transmission. This allows for a highly efficient use of and reduction in parts and for simplicity in the resulting vehicle. Alternatively, the transmission is sufficiently flexible that for some embodiments, the input shaft may be angled relative to the output shaft which can be accomplished by modification of the porting block between the pump head and the motor head of the transmission. In addition, an integral charge pump was determined to be desirable. The charge pump is driven by the input shaft which provides makeup hydraulic fluid to the pump head while allowing circulation of the fluid for cooling and filtering.